The present invention relates to an optical switching system, and more particularly to a wavelength division optical switching system and a communications network comprising such optical switching systems.
The recent introduction of optical fiber cables to communications networks have resulted in the need for developing optical switching systems which provide direct switching of light signals sent on optical transmission media. Due to the absence of the need for converting the light signals to electrical signals, optical switching systems are economically advantageous over electrical switching systems which are currently implemented by stored program controlled electronic switching systems.
Since different wavelengths of light can be regarded as a resource for creating transmission channels for carrying light signals, wavelength division multiplexing can multiply the number of channels that can be carried over a single optical fiber. Therefore, if wavelength division multiplexed light signals are transmitted between switching systems, it is desirable to make them directly handle such multiplexed signals to eliminate the need for wavelength division demultiplexers at the interface between a transmission line and a switching system.
A wavelength division optical switching system is described in Japanese Patent Publication No. 58-196796. The known optical switching system comprises optical multiplexers each multiplexing incoming light signals which are carried on different wavelength divided channels. The multiplexed signals are applied to a switching network which comprises an incoming wavelength switch, a space switch and an outgoing wavelength switch. The space switch comprises an array of crosspoints each having a plurality of space switch wavelength divided channels so that the space switch can operate in a wavelength divided fashion. Each of the wavelength switches includes an array of wavelength converters each of which converts the wavelength of a signal to a desired wavelength. A wavelength converter of the incoming wavelength switch converts the wavelength of a source terminal signal to an available space switch wavelength of a particular crosspoint. A wavelength converter of the outgoing wavelength switch converts the space switch wavelength to the wavelength of a destination terminal, so that the wavelength divided channel of the source station is switched to the wavelength divided channel of the destination station. The wavelength-switched multiplexed light signals are applied to an optical demultiplexer where they are demultiplexed into the terminals to establish a connection. Each of the wavelength converters can be implemented by a combination of a nonlinear optical device such as lithium niobate crystal for converting any of the wavelengths of the incoming signals to a fixed wavelength which is shorter than any of the wavelengths assigned to the terminals and an optical detector and a light-emitting diode (as described in article "p-n-p-n Optical Detectors and Light-Emitting Diodes", pages 810 to 813, IEEE Journal of Quantum Electronics, Vol. QE-14, No. 11, November 1978) for converting the fixed wavelength to any of the assigned wavelengths. The use of the lithium niobate crystal is due to the fact that the p-n-p-n optical detectors and light-emitting diodes are not capable of converting wavelengths to shorter ones.
However, the above-mentioned wavelength converter suffers from wavelength instability resulting from difficulty in strictly controlling the proportions of component materials within close manufacturing tolerances and further suffers from temperature dependent wavelength instability. Deviations of wavelengths from those intended are likely to produce crosstalk between adjacent wavelength divided channels, which could lead to a malfunction of switching operations.